Variable pitch reversible propeller



June 10, 1952 E. H. MORRIS ET AL- 2,600,017

VARIABLE PITCH REVERSIBLE PROPELLER Filed Aug. 21, 1947 2 SHEETS-SHEET 1 Ina/$71,257:;

ZZuarJE/Ybrrzls W a 0 b Patented June 10, 1952 VARIABLE PITCH REVERSIBLE PROPELLER Edward Hollingworth Morris, Ashtead, Ellis Danvers, Churchdown, Gloucester. and Thomas Edward Godden, Cheltenham, England, assignors to Rotol Limited, Gloucester, England, a

British company Application August 21, 1947, SerialNo. 769,894-

I In Great Britain June 25, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires June 25, 1966 7 Claims. 1

This invention concerns variable-pitch propellers of the kind in which the blades are capable of adjustment from a positive angle to provide forward thrust to a negative angle to provide backward thrusthereinafter referred to as reversible pitch propellers.

The object of the present invention i to provide a variable-pitch propeller of the kind referred to in which the pitch of the blades is automatically adjusted within the positive and negative pitch ranges.

According to this invention a. reversible, variable-pitch propeller comprises a pitch-change mechanism for adjusting the propeller blades, means for normally limiting the movement of the blades to within the positive pitch-range, a constant speed unit for automatically regulating the pitch-change mechanism in the positive and negative pitch ranges and a manual control for over-riding the constant-speed unit so that it operates the pitch-change mechanism to move the blades to within the negative pitch-range and for simultaneously removing the blade-movementlimiting means.

The manual control is preferably adapted also to reverse the sense of the control efiects on the pitch-change mechanism of the constant-speed unit when the control is actuated to bring the blades into the negative pitch-range.

The present invention finds particular application to variable pitch propellers in which the pitch-change mechanism is hydraulically operated for pitch-chang movements in both senses in which case the manualcontrol is adapted to bring into operation an over-riding hydraulic motor to over-ride the constant-speed unit and a reversing hydraulic motor to adjust a valve and thereby transpose the duct from the unit to the pitch-change mechanism.

A practical application of the invention will now be'described by way of example, with reference to the accompanying drawings whereof:

Figure 1 is a diagrammatic view, partly in section of the propeller and its control mechanism, and

Figure 2 is a fragmentary view of an alternative construction of a part of the mechanism of Figure l.

The propeller which is diagrammatically shown in the accompanying drawings is for use on a gas turbine engine in which the propeller, compressor and turbine constitute a common rotary system, the turbine being directly coupled to the compressor and being also coupled to the propeller through a vreduction gear. The drive from the turbine to the propeller is not shown in the drawings for the sake of clarity. 1

The propeller is generally indicated by the ref-. erence numeral 5 and is hydraulically operated for which purpose it has a pitch-change motor 6, the ram 1 of which is connected by a 8, a rod 9 and pin it] with each of the propeller blades I]. Hydraulic fluid for the operation 0! the pitch:- change motor 6 is provided by a pump 12 which supplies fluid under pressure to-the valve- I3 of a constant speed unit, generally indicatedby the reference numeral [4. The unit [4 is of known construction and comprises a flyweight; governor I5 which is provided to adjust th valve l3. The pressure fluid passes from the valve litothe motor-6. V f 7 There is also associated with the propeller 5 an electrically-driven feathering pump l6 which is adapted to provide fluid for the operation of the hydraulic motor 6 when the blades ll areto be moved to their feathered position or returned from said feathered position. I

The gas turbine engine is under thecontrol of the pi1ot, the speed, and hence the powerde livered by the engine, being regulated by means of a control I1 in the pilot's cockpit. V The power control I! is connected to a fuel valve '(not shown) of the gas turbine engine and is also connected through a suitable linkage ;l 8 with the axially adjustable sleeve l9 of {the unit-.14;

Movements of the linkage I8 will rotate the gear 20 which meshes with a rack 2| formed on the sleeve l9 so that the latter will be adjusted vertically to vary the datum setting of the con stant speed unit by adjusting the loading on the governor spring 22. It will be appreclatedthrefore, that when the control I1, is adjusted the quantity of fuel passing to the turbine engine and its speed are both regulated and in this way a de-- sired relationship is at all times maintained between speed and power of the engine. Simultaneously the datum setting of the unit ll'is varied so that it accords with the operating con-- The control I! is movable along a slot 23 from one end 0 corresponding to slow running of the turbine engine towards the position A which corresponds to full power. Intermediate'the positions A and C of slot 23 is a gate 25 which allows the control H to pass into the slot 24; and there-' after it is movable towards the end B'ther'eof;

3 The position B of the control 1 also corresponds to the fullpower'of the gas turbine engine.

Whilst the control I! is within 'slot 23 the propeller blades are Operating within their positive pitch-range so that the propeller thrust is ap plied to propel the aeroplane forwardly. When the control I! is in slot 24 the propeller blades are set Within the negative pitch-range as hereinafter described, and the propeller thrust is. applied as a braking force to the aeroplane.

The pump |2 is supplied with oil under pressure from the lubricating system of the turbine engine by a pipe 2-5. The pressure oil from the pump 2 is delivered to the valve by a-pipe and there is a branch 3| from this pipe which leads to a pair of valves 21 and '28. 5The'valve2l.

regulates the passage of pressure oil fromithe branch 3| to a motor 29, whilst the valve 28 regulates the passage of fluid from the branch 3| to a pair of motors 32 and 33.

"'The motor 29 is connected through a lost motion device 34 with the valve spindleof thec'onsta'nt speed unit I4 the arrangement being that when the motor 29 is'brou'ght into operation the unit-*isoverridden in'its operation. Accordingly, for convenience the motor'25 will be referred to as'the-overriding motor, its control valve 21 as thef ovierriding'valve and a solenoid 35 which controls the operation of the valve 2'! will be referred to as the overriding selenoid.

The'motor 33 is directly connectedwiththe valve '36 'whichis only brought into operation when it is- -desired to move the blades from one pitch range to the other during reversing or unreversingof the blades. For convenience the valve 36 is referred to as the reversing valve and-the motor-33 and valve 28 will thereiorebe referred-to as the reversing motor and reversing valverespectively. The valve 28 is actuated by a solenoid'tl "which will be referred-to hereinafter as the reversing solenoid.

The reversing valve 36 is'urg ed'by a spring 38 to assume a position in which the pitch adjustments are performed under the control of the unit 14 with the blades maintained in the positive pitch-range.

"It will be understood that 'when the reversing 111013017'33 is brought into operation the pressure fluid'whichis-supplied to it from valve 28 also passes to motor32. The latter is associated with areliefvalve -39 which closes a pipe line connecting the pipes 25 and 30 together. Therelief-valve 39 is set by spring 46 so that upon a predetermined delivery pressure being exceeded by "pump |2 oil passes from the delivery to the suction side of the pump. When the motor 32 is brought into operation it compresses spring 40 and the pressure at which the'relief valve'opcrates is increasedf0r this reason-the motor 32 will hereinafter be called the pressure-increasing motor.

There is ass'ociated'with the control ii a twoposition switch whose switch-arm 4| is adjustable to engage contact 42 by the control H as his moved throughthe gate 25 from the slot 23 tothejslot'24. Similarly. when the control I'l is moved through the gate25'from the slot '24 to the slot 23 the switch-arm 4| is moved away from contact 42 and brought into engagement with the contact 43.

The solenoid 31 is connected in series with a battery 44"and"with contact 42 so that when the switch arm 4| engages this contact the solenoidwill be'energised. The solenoid 35 is COIl-j necteddn series with the arm of a: two-position contact 41 is connected with contact 43.

4 switch 45 and in parallel across solenoid 31. The latter is adjusted by a pitch-varying device as later described. The contact 46 of switch 45 is connected to the contact 42 of switch 4| whilst With the arrangement described, when arm 4| engages contact 43 and arm 45 engages contact 41 solenoid 35 only is energised. Upon arm 4| engaging contact 42 and arm 45 engaging contact 45 the pair of solenoids .35, 31 are energised.

'The'arm 45 is adjusted by a slide 48 which is connectedlto .one arm of a bell-crank lever 49 the other arm of which carries a slipper 5|) to -.-engage with a circular plate 5| which rotates with the propeller 5. The plate 5| is axially ad- .j-ustable by means of a rod 52one of which is associated with each blade The rod 32 is spring urged towards the left and into engagement with a pin 53 carried by the root of the propeller blade The rod 52 and pin 53 constitute a lost motion mechanismso that inthe positive pitch-range of adjustment of the blades H the pin 53 does not engage the rod 52the switch 45 cannot, therefore, be actuated. When the propeller blades have reached afine pitch setting only a little above zero pitch the lost motion is taken up and the switch 45 will be under the control of the propeller. This for instance may occur at a pitch setting of about +5. As the blades of the propeller are movedinto the reverse pitch-range the slipper 48 is adjusted to move the switch arm. 45 towards engagement with contact 41. This will occur shortly. after the blades have movedinto the negative pitch range at say --.-5.

When the motor 5 is. brought into operation to move the blades 1 through the positive pitchrange towards the negative pitch-range the ram l is moved towards the left by pressure fluid from the motor |2 which is fed along thepipe -54 to behind the .ram 7. Pressure fluid from pipe '54 also passes along a duct 55 to theright hand end of valve 56. The latter is urged by a spring 5! towards the right. When the ram 1 has-set the blades H to approximately the zero pitch position a cylindrical extension 58 of the ram 1 engages'with aseries of oircumferentiallyspaced stops 59. The latter are constituted by enlargements of resilient fingers 60 which are carried by a ring 6|. .The fingers 60 are designed so as normallyto collapseradially inward but are maintained against such movement by an abutment 64 which is capable .of axial movement towards the right against spring 62. Such an arrangement of stop for the ram of the pitchchange motor is known and need not be described in greaterdetail.

When the cylindrical portion 58 of the .ram 1 engages the stops 59 the .ram is broughtto rest but if the pump l2 continues to deliver pressure fluid through pipe 54, the hydraulic pressure within the motor 6 rises. The increased pressure is communicated by conduit. 55 to the valve 56 and upon'the pressure attaining a predetermined value the valve 56 is moved against its spring 51 towards the left. Thispermits the pressure fluid to passthroug'h the valve to a passage 63which leads to the left hand end of the cylindrical abutment B4. The latter is thereupon moved towards the right againstthe action of itsspring 62.nand clear of the fingers 59. The latter will therefore collapse radially inwards and permit the ram 1 further movement towards therleft tobring the blades to within negative pitch-range.

'fiubsequently when the blades 1 have beenbrought into the positive pitch-range the stops 59 are returned byabutment 64 to lie in the path of the part 58 of the ram I.

The stops 59 become effective to prevent further movement of ram I towards the left when the blades are at zero pitch.

The sequence of operations performed under different flight conditions will now be described. Prior to starting the power plant it is assumed that the ram I of the pitch change motor 6 is against the stop 59 (the blades being at zero pitch) and that the control II is adjusted to its slow-running position C. The power plant is then started.

For take-01f the control II is moved to its fullp-ower position A, i. e., with forward thrust, and as a consequence the power plant accelerates. The blades II remain at their zero-pitch setting until the speed of the plant exceeds the datum speed for which the unit I4 has been adjusted. Thereafter the blades II are adjusted by motor 6 towards a coarse-pitch position until the full power of the engine is absorbed. Whilst the blades II are being thus adjusted ram I moves clear of the stops 59.

During normal flight, control I! is adjustable to any position along slot 23 and the pitch of the blades II controlled by the unit I4 within the positive-pitch range, the datum setting of the unit being made to accord with the power selected by lever II.

During the landing approach the power control I! is adjusted to the minimum power position for landing i. e. to about opposite gate 25, the lever I'I remaining within slot 23. The datum setting of unit I4 will be correspondingly adjusted at the same time and it is arranged that power developed by the engine under these conditions is suflicient to provide only a proportion of the power required to drive the propeller. As a consequence the propeller motors and acts as a windmill to provide the remaining portion of the power. This will have the efiect of producing drag thereby preventing the gliding speed of the aircraft from becoming excessive. If the pilot wishes to reduce this drag or. to obtain a thrust in the forward direction he suitably adjusts the lever II away from its minimum-power position and towardsthe end A of slot 29.

As soon as the aircraft touches down control II is moved through gate 25 from the forwardthrust slot 23 to slot 24 and is then moved to its maximum power position B. During the movement of the power control from the slot 23 to the slot 24 an arm M of the associated two-position switch is moved to engage the contact 42. This completes a circuit from battery 44 to solenoids 35 and 31 and both are energised. Actuating solenoid 31 sets valve 28 to pass pressure fluid to motors 32 and 33 and as a consequence the connections from valve I3 to motor 6 are transposed; at the same time the blow-ofl pressure of valve 39 is increased. Actuation of solenoid 35 sets valve 21 to pass pressure fluid to motor 29 so that valve I3 is overridden to direct pressure fluid from pump I2 by pipe 54 to the motor 6; ram I is thus urged to the left i. e. to move blades II into the negative-pitch range. The motor 32 in adjusting valve 39 permits the pump I2 to deliver at an increased pressure to motor 9 so that valve 56 is opened. The abutment 64 is thus moved to the right allowing stops '59 to fall inwardly-the ram I is then free to continue its travel and the blades enter the negative-pitch range.

When the blades have assumed a position corresponding to a small negative pitch (atabout 5) the slide48 of the pitch following device moves arm 45 of its two-position switch on to stud 4! to break the circuit to the overriding solenoid 35. Solenoid 31 remains energised, however, so that the oil duct connections from the valve I3 of unit I4 to the motor 6 remain transposed. The overriding solenoid being de-energised, the unit I4 is free to assume control again and due to its transposition the propeller is able to constant :speed within the negative-pitch range.

.The amount of braking thrust obtained is controlled by adjusting the position of control I'I along the reverse-thrust slot 24 as its movement away from gate 25 towards the end B of the slot has no efiect on the switch 4|.

The minimum power position to which the con trol I1 maybe set with reverse thrust is arranged tobe not less than that required to maintain the datum speed for which the unit I4 has'been set by the control II.. This will ensure that the propeller blades remain within the negativepitch range. If the power-were to be reduced below this value the speed would fall and as a consequence the propellerblades would be moved towards the positive pitch range until thisaction was stopped by the pitch-following device adjusting switch 45 to energise .the overriding solenoid 35 once again.

When the landing run is complete the power control I1 is moved to its slow-running position C. In performing this movement arm 4| is set on to stud 43; the overriding solenoid 35 is energised and simultaneously solenoid 31 is deenergised so that the unit I4 is overridden and pressure fluid is directed along pipe 64 to the left hand end of the motor 6 to move ram I to bring the blades II into the positive pitch range. Finally, the overriding solenoid is cut-out when the two-position switch 45 is actuated by the pitch-following device on the blades being returned to the positive pitch range.

In the modification shown in Figure 2, the unit I4 is normally adjusted by the control I! as described above but in this arrangement it is capable of being overridden in its operation by a manual speed control III. The latter may be set to three positions one of which (D) corresponds to the normal operation of the propeller 5 when the unit I4 is adjusted by the control I! in the manner described above. Another position (E) corresponds to a setting of the propeller blades at which the maximum speed of the power plant is obtained and the third position F is selected when it is desired to feather the blades. When the speed control I9 is set to position E or F it overrides the control I! in adjusting the unit I4. Control I0 is connected with unit I4 by a linkage 76 for this purpose.

In contradistinction to the arrangement of Figural the valve 28 controls the passage of pressure fluid to the reversing motor 33 only whilst the valve 2! regulates its passage to both the overriding motor 29 and the pressure-increasing motor 32.

There is associated with the control III a switch II which is only closed when the control is set to position E. Similarly there is associated with control I! a two-position switch I2. The latter is actuated by the control I! in the same manner as described with reference to Figure 1, namely, the switch is adjusted to open one circuit and close another when the control I! is moved from slot 23 to 24 and vice versa.- Switch II is con;

Figure 1.

nected in series with overriding solenoid 35, battery M, and a switch "14 associated with a pitch following device generally indicated by the referencenumeral 13. One side of switch 12 is connected in series with overriding solenoid 35, battery 44 and anotherswitch 15 associated with the device 13. The. other side of switch 12 is connected in series with the battery A l and with the reversing solenoid 37. The device 73 is operated. by the blades Ii through a lost-motion mechanism in the. same way. as described. for

The pitch-following device issconmated. with the switches l4, it so that switch "I l is opened when the blades :reach a pitch-selected iortake-off (the take-off: pitch) and switch T15 is actuated when the blades are :set to a small negative pitch.

When the motor 6 is brought into operation to-move the. blades II from the positive to the negative pitch range :the :ram. 1 'is'stopped when thebblades reach their zero-pitch position as describled'with reference .to Figure '1.

Thecontrols J1 and 1.8 are interconnected so thatthe speed control must be moved to its maximum speed position before the control l'i can-the movedfrom slot 23 to slot 24 and further so that control ll cannot be moved to the slowrunning position C whilstcontrol it is at its maximum speed position. E.

The sequence of operations performed with the arrangement of Figure 2 under different flight conditions will now be described.

Prior to starting the engine it is assumed that the ram" lisiagainst the zeropitch stop, .that the power control H is in its slow-running position and that thespeed control It is in its normal position. The power plant is then started.

For take-ofithe' power control I! is moved'to position A and .as a consequence the engine accelerates. The blades it remain at their zeropitch setting until the speed of the engine ex ceeds the datum speed value for which unit 44 is adjusted whereupon the blades are moved towards a coarse position until the full power of the engine is absorbed.

During normal flight the power control ll may be adjusted to any position along slot 23 i. e. for forward thrust and the propeller blades are adjustable by the constant-speed unit at settings above the zero-pitch stop.

1 During the landing approach the power control I1 is moved to lie opposite gate 25, a position corresponding to minimum power for landing. The speed control "is is thereafter moved to its maximum speed position (E) and as a consequence switch H is closed. The efiect of this is to energise the overriding solenoid 35 so as to permit hydraulicfiuid to enter motor 6 to move the blades ii towards the negative-pitch range. When the blades are thus moved to a duel pitch setting only a little above zero pitch the pitchfollowing device 13 is operated to open switch M and break'the circuit to the overriding solenoid 35. The propeller is then free to constant speed at the maximum speed selected by the manual speed control. Under these conditions the engine power is sufficient to provide only a proportionof the maximum speed demanded by the speed control Hi-and as a consequence the propeller must act as a windmill to provide the remaining portion of the power. This will have the effect of producing drag thereby preventing the gliding speed of the aircraft from becoming excessive. If the pilot wishes to reduce this drag or-to obtain a thrust in the forward direction 8 he suitably adjusts the power control lever ll towards the full power position A.

As soon as the aircraft touches down thepower control I1 is moved sideways through gate 25 and is then moved to its maximum power position B with reverse thrust. During the movement. of the power control from slot 23 to slot 2d the twoposition switch 72 is actuated so that solenoid 35 is again energised; as a consequence'the zeropitch stop is withdrawn and unit i4 is overridden so that the propeller bladesare moved to within the negative-pitch range. When the blades have assumed a position corresponding .to a small negative pitch the pitch-following device 753 actuates switch 15 to break the circuit to'solenoid 35 and to energise the reversing solenoid 31. As a consequence the reversing valver3t is positioned to transpose the oil ductconnections from the unit [4 to motor 6 and the. propeller 5 is then free to constant speed within the reverse pitch-range.

The amount of braking thrust. obtained. is varied by adjusting the position of the power control I? along slot 25.

The minimum power position to which the power control may be set with reverse thrust is arranged so that the power cannotbe reduced below that required to maintain the maximum speed selected by the manual control 'Tilwith. a small amount of reverse thrust. This is'toensure that the propeller blades remain within the reverse-pitch range. If the power were to be reduced below this value the speed would fall and as a consequence the blades H would be moved towards the positive-pitch range until this action was stopped by the adjustment of switch '15. by the pitch-following device.

When .the landing run is completed the speed control iii is returned to position D andthe-power control is moved to its slow-running position 0. This movement of the power control .actuates switch 72 and brings the overriding solenoid-L35 into operationto permit pressure fluid to pass to the reversing motor 33 so that the reversing valve is returned toits normal position. Finally. the reversing and'overriding solenoids are out out by switch it when the blades are about to enter the positive pitch range; the propeller blades then assume a position ready for taxiing or take-off.

We claim:

1. A variable pitch propeller comprising a hub, at least two blades carried for rotation with the hub and capable of pitch-change movements within a positive and negative pitch range, an hydraulic pitch-change motor, a transmission between the motor and each bladewhereby the latter is adjusted by the motor, a mechanical'stop to prevent the blades moving into the negative pitch range, an hydraulically-operated means to remove the stop and thereby allow the blades to move into and within the negative pitch range, a pump to supply the motor with fluid under pressure, a constant speed unit automatically to regulate the passage of pressure fluid from the pump to the motor with variations in propeller speed when the blades are in the positive and negative pitch ranges, a valve to reverse the hydraulic connections from the constant speed unit to the pitch change motor, means-temporarily to over-ride the constant speed unit to pass pressure fluid to the motor, a manual control to adjust the reversing valve and temporarily to "set the over-riding means of the constant speed unit so that the blades are moved to within the negative pitch range and blade-controlled means to re- .9. set the over-riding means when the blades are brought within the negative pitch range so that the blade pitch is automatically adjusted by the constant speed unit within the negative pitch range.

2. A variable pitch propeller as claimed in claim 1 wherein the mechanical stop is normally effective to prevent movement of the ram of the pitch change motor, said stop being displaceable to permit further-movement of the ram to bring the blades into the negative pitch range by an hydraulic, motor which is subjected to the pressure fluid in the pitch change motor.

3. A variable pitch propeller comprising a hub, at least two blades carried for rotation with the hub and capable of pitch-change movements within a positive and negative pitch range, an hydraulic pitch-change motor, a transmission between the motor and each blade whereby the latter is adjusted by the motor, a stop to prevent the blades moving into the negative pitch range, and hydraulically-operated means to remove the stop and thereby allow the blades to move into and within the negative pitch range, a pump to supply the motor with fluid under pressure, a constant speed unit automatically to regulate the passage of pressure fluid from the pump to the motor with variations in propeller speed when the blades are in the positive and negative pitch ranges, a valve to reverse the hydraulic connections from the constant speed unit to the pitch change motor, means temporarily to over-ride the constant speed unit to pass pressure fluid to the motor, a manual control to adjust the reversing valve and temporarily to set the over-riding means of the constant speed unit so that the blades are moved to within the negative pitch range and blade-controlled means to reset the over-riding means when the blades are brought within the negative pitch range so that the blade pitch is automatically adjusted by the constant speed unit within the negative pitch range.

4. A variable pitch propeller comprising a hub, at least two blades carried for rotation with the hub and capable of pitch-change movements within a positive and negative pitch range, an hydraulic pitch-change motor, a transmission between the motor and each blade whereby the latter is adjusted by the motor, a mechanical stop to prevent the blades moving into the negative pitch range, an hydraulically-operated means to remove the stop and thereby allow the blades to move into and within the negative pitch range, a pump to supply the motor with fluid under pressure, a constant speed unit automatically to regulate the passage of pressure fluid from the pump to the motor with variations in propeller speed when the blades are in the positive and negative pitch ranges, a valve to reverse the hydraulic connections from the constant speed unit to the pitch change motor, means temporarily to over-ride the constant speed unit to pass pressure fluid to the motor, a manual control to adjust the reversing valve and temporarily to set the over-riding means of the constant speed unit so that the blades are moved to within the negative pitch range, blade-controlled means to re-set the overriding means when the blades are brought within the negative pitch range so that. the blade pitch is automatically adjusted by the constant speed unit within the negative pitch range, said pitchchange motor comprising a movable ram, said mechanical stop being normally effective to prevent movement of said ram, an hydraulic motor subjected to the pressure fluid in the pitchchange motor for displacing said stop to permit further movement of the ram to bring the blades into the negative pitch range, a relief valve for limiting the pressure of fluid passing to the pitchchange motor, and means for increasing the loading of the relief valve when the manual-control is adjusted to bring the over-ridingmeans into operation.

5. A propeller as claimed in claim 4 wherein the loadingof the relief valve is increased by a hydraulic motor. a I

6. A variable pitch propeller comprising a hub, at least two blades carried for rotation with the hub and capable of pitch-change movements, Within a positive and negative pitch range, an hydraulic pitch-change motor, a transmission between the motor and each blade whereby the latter is adjusted by the motor, a mechanical stop to prevent the blades moving into the negative pitch range, an hydraulically-operated means to remove the stop and thereby allow the blades to move into and within the negative pitch range, a pump to supply the motor with fluid under pres- .sure, a constant speed unit automatically to regulate the passage of pressure fluid from the pump to the motor with variations in propeller speed when the blades are in the positive and negative pitch ranges, a valve to reverse the hydraulic connections from the constant speed unit to the pitch change motor, means temporarily to over-ride the constant speed unit to pass pressure fluid to the motor, a manual control to adjust the reversing valve and temporarily to set the over-riding means of the constant speed unit so that the blades are moved to within the negative pitch range, blade-controlled means to reset the over-riding means when the blades are brought within the negative pitch range so that the blade pitch is automatically adjusted by the constant speed unit within the negative pitch range, hydraulic means for adjusting the overriding means and the reversing valve, solenoidoperated valves for controlling the pressure fluid to the hydraulic motors, and switches for controlling the solenoid-operated valves and being adjusted by the manual control and by the bladecontrol means.

'7. A variable pitch propeller comprising a hub, at least two blades carried for rotation with the hub and capable of pitch-change movements within a positive and negative pitch range, an hydraulic pitch-change motor, a transmission between the motor and each blade whereby the latter is adjusted by the motor, a mechanical stop to prevent the blades moving into the negative pitch range, an hydraulically-operated means to remove the stop and thereby allow the blades to move into and within the negative pitch range, a pump to supply the motor with fluid under pressure, a constant speed unit automatically to regulate the passage of pressure fluid from the pump to the motor with variations in propeller speed when the blades are in the positive and negative pitch ranges, a valve to reverse the hydraulic connections from the constant speed unit to the pitch change motor, means temporarily to over-ride the constant speed unit to pass pressure fluid to the motor, a manual control to adjust the reversing valve and temporarily to set the over-riding means of the constant speed unit so that the blades are moved to within the negative pitch range, blade-controlled means to re-set the overriding means when the blades are brought within the negative pitch range so that the blade-pitch is automatically adjusted by the constant speed 

